Chapter 5

Corrosion Protection Protection Theory

Metal or alloy + corrosive environment = corrosion

Metal or alloy + barrier + corrosive environment ≠ corrosion

Barriers Sacrificial Barrier Inert Barrier Perfect Barrie Sacrificial Barrier

 Metallic barrier using a more active metal than the one required to be protected  Example: Steel protection by Zn, Al … Inert Barrier

 Metallic  Using LESS active metal  Example: for steel protection by Cu, Ag.  Non-metallic  Example: Paints, cement, glass, ceramic, plastic, polymer and paints. Perfect Barrier

 Alloys like stainless steel are protected by chromium, nickel added to the alloy.  The protection is inside the grain Protection Methods

 Coatings  Inhibitors  Cathodic protection Coatings

Types of coatings  Metallic Coating  Non – metallic Coating  Plastics  Paints Metallic Coating

 LESS active metal such as Cu, Ni for steel protection  MORE active metal such as Zn, Al for steel protection

Galvanization =(Hot dipping) Hot Zn (fusion of coating) to protect steel.

Electroplating Spray Vapor Deposition

Non – metallic Coating

 Cement  Carbon  Ceramics  Glass(alumina for high temp) All these materials corrosion resistance. plastics

 Polymer  Epoxy  PVC  PVA  PE

Paints

 Paints are composed of:  1. Pigments  2. Binder 95% of paints are (polymer + oils)  3. Solvent Water based solvent or Oil based solvent

Properties to consider the quality of coating

 Average thickness of coating  Porosity or continuity  Adherence  Uniformity of thickness  Inert or passive Cathodic Protection

 For a corrosion cell containing anode and cathode it will provide positive corrosion current and by applying negative current equal or more than the positive corrosion current, all anodic area will be converted into cathodic area .  Here metal is protected by cathodic protection methods. Methods of cathodic protection Sacrificial anode method. Impressed current method. Sacrificial Anodes Method

The main idea is connecting the metal to be protected to more active metal. To protect steel we can use Zn, Al, Mg……

Protection of Steel Structure

 By connecting a piece of Zinc or other anodic material to the steel, in this case, the zinc or other anodic metal is termed “Sacrificed Anode“.  It is called a sacrificial anode . Cathodic protection of an underground pipeline

Cathodic protection of an underground pipeline using a zinc sacrificial anode

Cathodic protection of a ship hull

Cathodic protection of a ship hull using a zinc sacrificial anode

Impressed current Method

 This method of protection is carried out by making the metal to be protected a cathode by connecting it to the negative pole of a DC power supply (electrolytic cell).  This is achieved by using an external power source.  -ve is joined to the metal to be protected(cathode)  +ve is joined to an inert anode (Pt, graphite or metal scrap (Fe))